Transdermal application of carbimazole, a safe and efficient treatment of feline hyperthyroidism?

Abstract

Methimazole and carbimazole, two blockers of the thyroid hormones synthesis, are used for the medical management of feline hyperthyroidism. Mostly pills are used, but in the Netherlands a carbimazole-ointment is produced for non-cooperative cats to administer in the inner pinnae of the ears. Our first goal was to find out the way of absorption, via transdermal or oral route. In the first part of the study the transdermal absorption of this ointment is studied in six healthy cats. During the first treatment week potential grooming of the cats was inhibited by wearing an e-collar. After a week of washout the spreading of the ointment was tracked for another treatment week by treatment with a dye-coloured ointment without the e-collar. The cats were both weeks treated with 5 mg carbimazole every 12 h, alternating the nonhaired portion of the left and the right inner pinna. Serum methimazole concentrations were measured on day 1 (t=1, 6 and 12 h) and day 7 (t=1, 6 and 12 h) in the two treatment weeks. One extra blood sample was taken on t=0 of day 1 in the second treatment week. The average serum methimazole concentrations showed a large variation. The average concentration found on day 7 at the third blood sampling (12 h) while the cats were not wearing an e-collar (0.378±0,227 μg/ml) differed significantly from the same sampling times on day 7 (0.167±0.174) and day 1 (0.108±0.077) with e-collar. Also, a spreading of the dye during the week without e-collar was visible at the tongue and around the medial side of both carpi. These results suggest an oral uptake of the ointment with more enteral absorption above transdermal absorption. The other part of the study involved patient visits and interviews. Fifty-seven owners of hyperthyroid cats have been interviewed and twelve of these cats have also been observed after administration of the ointment. The aim of this part was to study the efficacy of the ointment and safety for the (skin of the) owners. All owners saw improvement during treatment with the ointment. Only five owners noted their cat washed the ointment out of the inner pinnae, which would suggest transdermal absorption of the ointment. Twenty cats had changes in the inner pinna as crustae and redness, in nine cats this disappeared while continuance of the treatment. Six of the fifty-seven owners experienced skin problems as redness and itchiness during the treatment with carbimazole. Because of possible skin problems during treatment with carbimazole we recommend to carefully instruct the owners about animal handling, application of the ointment and own skin health while treating the pet.